Perhaps you have added invalid code or placed it in the wrong part of the page?
- Remove the code or make sure it's 100% valid and does cause conflicts with other parts of objects on the page.
Related tutorial: http://www.wysiwygwebbuilder.com/add_html.html

Did you use any unofficial extensions?
Some unofficial extensions may cause conflicts with standard elements. Try to remove the extension(s) and then republish the page.